Conventionally, a fault current limiter using a superconductor is known (for example, see Japanese Patent Laying-Open No. 2006-278135 (PTD 1)). The fault current limiter disclosed in PTD 1 includes a circuit in which a superconducting fault current limiter module having a superconductor and a switch are connected in series, and an external resistor connected in parallel to the circuit. In the fault current limiter, as the superconducting fault current limiter module performs a current limiting operation triggered by an excessive fault current, the current is bypassed to the external resistor, and meanwhile the switch is opened to stop the current supply to the superconducting fault current limiter module.
The fault current limiter described above performs the current limiting operation as the superconductor is quenched by a fault current, however, the temperature of the superconductor rises due to the fault current during the current limiting operation. In the fault current limiter described above, in order to prevent the temperature of the superconductor from rising excessively during the current limiting operation, the switch is opened to stop the fault current from flowing to the superconductor; however, in the case where the switch malfunctions or depending on the conditions under which the fault current has occurred, the temperature of the superconducting may rise rapidly to about 500K, for example.
Affected by the temperature rise of the superconductor, the temperature of a coolant (for example, liquid nitrogen) used to cool the superconductor also rises and reaches a boiling state. When the heat is weak, the boiling state remains at a nucleate boiling state where small bubbles are generated continuously; however, as the heat becomes greater than a critical heat flux for nucleate boiling, the boiling state transits to a film boiling state in which the superconductor is being covered by a film of great bubbles, and the temperature of the superconductor jumps discontinuously to a high temperature. It has been reported that in the film boiling state, the heat flux transferred from the superconductor to the coolant (liquid nitrogen) is only about 15% of the critical heat flux for the nucleate boiling state (for example, see “Transient heat transfer from a silver sheathed high-Tc superconducting tape in liquid nitrogen”, Masahiro Shiotsu et. al., Proc. of the 16th International Cryogenic Engineering Conference/International Cryogenic Material Conference, Vol. 1, p. 617-620, 1997 (NPD 1)). The reason therefor is that in the film boiling state, the superconductor is being covered by a film of great bubbles, and thereby the superconductor is thermally insulated from the surrounding coolant.